It would probably be more efficient tax-wise to do some combination of realizing income in the COBRA year and realizing income the subsequent five or six years.
So for example, realize $250K worth of income in the COBRA year, buy five $50K CD's in ladder formation, and realize $20K worth of income (by additional 401(k) withdrawals) each of those subsequent five years.
Evening out the withdrawals helps you in two ways. First, it minimizes your exposure to the higher tax brackets - those $20K withdrawals will probably be taxed more lightly (and later in time) than the $100K would be if it were part of an initial $350K withdrawal.
Second, in order to qualify for ACA tax benefits, your AGI generally needs to be above 100% or 138% of the FPL, depending on the state you live in. Your FPL in turn depends on your tax family size which is generally the total number of exemptions you can claim on your taxes (line 6c of 1040 IIRC). Realizing that $20K of income helps you generate the AGI you need to be on ACA instead of Medicaid or full-freight insurance.
If it were me, I would figure out what AGI I was aiming for (personally I aim for 199% FPL) in those five years, subtract the interest I'd get on the CD's from that number, and then target that amount for the five smaller 401(k) withdrawals. Once you know that amount, you can figure out what your annual shortfall is to your budget (presumably $70K), multiply that by 5, and then use that for the initial large COBRA year withdrawal.
Good luck.